Close approach of C/2013 A1 (Siding Spring) to Mars

I have been thinking about how water was once on the surface of Mars. Today there is no evidence on water on the surface only 'channels' of were water once flowed.

I guess, water is still on mars but under the ground.

Could an asteroid have brought the water up to the surface, then after the asteroid has passed the water sinks back under the subsurface of mars ?

If a massive asteroid passed close by the planet mars, could the gravitational pull of the asteroid exert a pull on body of mars, that could have an effect on mars molten core centre, exerting heat and pressure upwards, that brings mars water liquid to the surface, then the water on mars sinks under the ground after the effect of the asteroids pull has passed.

I though of this after thinking about how the Earths moon gravitational pull effects the earth by moving water to create tides and the effects on the earths core. And how the gravitational pull of Jupiter has effected Europa's core, creating heat and pressures at the core to exert the water to the surface.

Do you think this close encounter with Comet Siding Spring would have a gravitational pull that would bring any water up from the subsurface of mars.

There is still water on the surface of Mars, the problem is that it is all frozen rather than liquid. The polar ice caps of Mars contain substantial amounts of water ice, along with frozen carbon dioxide. Similarly there is water ice in the subsurface of Mars at high latitudes, much like the Arctic permafrost on Earth.

Tidal heating can be an important effect, and is indeed what keeps Europa's subsurface ocean liquid. To have that kind of tidal effect though needs a very massive body, Jupiter and the other Galilean moons in the case of Europa, and the Moon in the case of Earth. By comparison Comet C/2013 A1 (Siding Spring) is tiny and the tidal pull it will exert on Mars would not be noticeable. Tides do work both ways however, and the comet will experience quite large tides from Mars, which could significantly affect the structure of the comet.

Large impacts on the other hand might be able to temporarily melt some of the permafrost. In the case of C/2013 A1 (Siding Spring) we know that an impact is very unlikely, but Mars does get hit by large objects every so often.